In the world of American historians, Thomas Jefferson is considered a tad overexposed. That's why Joseph Ellis' 1997 National Book Award- winning American Sphinx was such a coup. Here was the familiar Jefferson--egalitarian aristocrat, slaveholding author of the Declaration of Independence, globetrotting homebody--plumbed one step further. Ellis used his empathic powers to convey how Jefferson explained himself to himself--as a young idealist constructing "interior worlds of great imaginative appeal," even if they didn't jibe with reality, and later on keeping his contradictions alive with an "internal ability to generate multiple versions of the truth."

Now readers are left wondering just how deep Ellis' empathy runs. Last week Ellis, an icon on the South Hadley, Mass., campus of Mount Holyoke College, where he has taught for the past 29 years, was exposed as an inventor of his own world of imaginative appeal. For several years Ellis has been using tales from his days as a platoon leader and paratrooper in Vietnam to help animate his popular course Vietnam War and American Culture. As his fame increased, Ellis began sharing his war stories with journalists and colleagues. The problem is that Ellis never served in Vietnam, as the Boston Globe disclosed last week. He rode out the war doing graduate work at Yale and teaching history at West Point.

Why did Ellis lie? For the most part, his stories weren't heroic but put him--and through him, his students--on the scene. It was sometimes a florid stage, as when Ellis told of seeing a burly comrade reading Emily Dickinson and weeping on the battlefield. "There is a classroom persona you have as a teacher that's not quite you," says Mount Holyoke's dean of faculty, Donal O'Shea. "There's an element of great teaching that's theater. And Professor Ellis was expert at that." Fellow baby boomers speculate that Ellis gave in to a generational tendency to exaggerate one's part in the great events of the 1960s.

Ellis has offered no explanations. "Even in the best of lives, mistakes are made," he said in a written statement. At a lecture in Washington last week, he added that he "deeply regret[ted]" his dishonesty and would now focus on "my own personal shortcomings."

Mount Holyoke will too. As part of a formal investigation, students who took Ellis' Vietnam course will be contacted to determine the severity of the lies. Some colleagues suspect that Ellis will resign before the investigation is complete. "He's devastated by this," says O'Shea. Academics, and historians in particular, traditionally think of truth as their gospel and the classroom as their church. "Knowingly being dishonest in class is just as great an act of moral turpitude as being knowingly dishonest or inaccurate in your written work," says David Garrow, a Pulitzer prizewinning historian at Emory University.

And what about that written work? Ellis' wildly popular Pulitzer prizewinning Founding Brothers, now in its 15th printing, has made him one of the most widely read historians not named Stephen Ambrose. Having cultivated a thriving general readership, he must now reckon with legitimate questions about whether his truth bending extended to his scholarship. Ashbel Green, Ellis' editor at Knopf, says the publishing house supports him and will work with him in the future. "I know these things aren't easy to compartmentalize," says Green, "but no one has questioned his scholarship in any of the books he has written." At least, not until now. Says Robert Nylen, a friend of Ellis' and a former professor at Smith College: "You just know there's going to be a tribunal of self-appointed thesis students poring through his work looking for every little mistake."

Certainly there is no shortage of professional jealousy among historians, and crossover acts like Ellis' tend to attract a particular amount of scorn. But some historians believe that Ellis' stumble could serve as a valuable lesson to his public. "Readers have to understand that whatever objective claims historians may make, they are invested in the things they write," says Michael Zuckerman, a history professor at the University of Pennsylvania. "History can't just be the production of the historian. It has to be the collaboration of a skeptical historian and certainly a skeptical reader."

With reporting by Anne Moffett/Washington and Andrea Sachs and Rebecca Winters/New York

Ellis, Revised

Among the professor's fabrications:

--That he performed active duty in Vietnam as a platoon leader and paratrooper

--That his unit was nearby when American soldiers massacred hundreds of Vietnamese civilians in the village of My Lai

--That he served on the staff of the U.S. commander in Vietnam, General William Westmoreland